Transposition table

A transposition table, or TT, is memory used by a chess engine so it does not have to calculate the same position from scratch every time it appears. A transposition occurs when different move sequences lead to the same position, with the same side to move and the same conditions that affect which moves are legal.

The table is usually implemented as a hash table, a data structure that turns the essential information about a position into a compact key that can be located quickly. An entry may store search depth, a measure of how far the engine explored from that position, a score describing how favorable the position appears or a bound on that score, and a good move found there. When the position appears again, the engine can reuse that information to avoid redundant work or to investigate moves that already look promising first.

A TT is temporary and finite, so entries are replaced as the search continues. It is not an opening book or a tablebase. Those resources contain information prepared in advance; a transposition table stores results produced by the current search. In many interfaces, the setting called Hash controls how much of the computer's working memory is allocated mainly to this table.